1. Field of the Invention
The present invention relates to an image reading apparatus for reading a reflective original and a transmissive original.
2. Related Background Art
The original image reading apparatus capable of reading both a reflective original and a transmissive original is disclosed for example in U.S. Pat. No. 4,989,099. However, this apparatus can only read a reflective original or a transmissive original at a time. More specifically, the apparatus is equipped with a light source for the reflective original and a light source for the transmissive original, and reads the reflective original by lighting the former light source or the transmissive original by lighting the latter.
Such reading apparatus is not capable of reading an original having both a reflective area and a transmissive area, or of simultaneously reading a reflective original and a transmissive original placed on the original supporting table.
Also it is already well known to effect, in case of reading a reflective image only, shading correction on the signal read from the original image, based on shading data obtained by reading a reference white board.
However, in case of reading an original containing a reflective area and a transmissive area therein, satisfactory image reading cannot be attained even if the conventional shading correction is conducted.
An example of such original containing a reflective area and a transmissive area is an OHP (overhead projector) film. Such OHP film original is usually stored in a framed state as shown in FIG. 12A, and it is required either to exactly read the image of the film area inside the frame or to read the image inclusive of the information written on the frame.
In case of simultaneously reading the film area and the frame area, the image reading has to be executed respectively by the transmitted light and the reflected light, but the states of shading for these two image areas are completely different as shown in FIG. 12B.
For example, at a sub-scanning position A in FIG. 12B, as the image is read by the reflected light, there is required shading correction for the reflected light. At a sub-scanning position B, there are required shading correction for the reflected light in the main scanning areas B1 and B3, and shading correction for the transmitted light in the main scanning area B2.
However, the conventional original reading apparatus has been unable to obtain a satisfactory image in this manner. The same applies to the case of simultaneously reading an image projected from a 35 mm film and a reflective original image.